A magnetic random access memory (MRAM) is expected and actively developed as a nonvolatile memory capable of performing a high-speed operation and rewriting an infinite number of times. In the MRAM, a magneto-resistance effect element is integrated in a memory cell and data is stored as a magnetization direction of a ferromagnetic layer in the magneto-resistance effect element. Some types of MRAMs are proposed to meet methods of switching the magnetization direction of the ferromagnetic layer.
A current-induced magnetic field writing MRAM is one of the most general MRAMs. In this MRAM, a wiring line that a write current passes through is installed on the periphery of a magneto-resistance effect element and a magnetization direction cf a ferromagnetic layer in the magneto-resistance effect element is switched by a current-induced magnetic field that is generated by the write current passing by. This MRAM can theoretically perform writing at a speed of 1 nano-second or less and thus, is suitable for a high-speed MRAM. For example, a success of an operation at 250 MHz is demonstrated in one report (N. Sakimura et al., “A 250-MHz 1-Mbit Embedded MRAM Macro Using 2T1MTJ Cell with Bitline Separation and Half-Pitch Shift Architecture”, Solid-State Circuits Conference, 2007, ASSCC' 07, IEEE Asian p. 216). Further, a circuit configuration suitable for an operation at 500 MHz is proposed (N. Sakimura et al., “MRAM Cell Technology for Over 500-MHz SoC”, IEEE JOURNAL OF SOLID-STATE CIRCUITS, Vol, 42, 2007, p. 830).
However, a magnetic field for switching magnetization of a magnetic body securing thermal stability and resistance against external disturbance magnetic field is generally a few dozens of [Oe]. In order to generate such magnetic field, a large write current of about one or a few mA is needed. When a write current is large, a chip area is necessarily large and power consumed for writing increases. In addition, when a size of a memory cell is miniaturized, a write current further increases and is not scaling. The technique is desired that a write current can be reduced according to miniaturization of a size of a memory cell.
As a wiring method that can suppress an increase of the write current according to the miniaturization, a “spin transfer method” is proposed (see Japanese Patent Publication JP2005-93488A and J. C. Slonczewski, “Current-driven excitation of magnetic multilayers”, Journal of Magnetism & Magnetic Materials, 159, L1-L7 (1996)). According to the spin transfer method, a spin-polarized current is injected into a ferromagnetic conductor and magnetization is switched due to a direct interaction between spin of conduction electrons that bear the current and the magnetic moment of the conductor (hereinafter referred to as “spin transfer magnetization switching”). Generating of the spin transfer magnetization switching depends on a current density (rather than a current absolute value). Accordingly, when the spin transfer magnetization switching is utilized for data writing, as the size of the memory cell decreases, the write current is also reduced. In other words, the spin transfer magnetization switching method is excellent in scaling performance. When the write current is small, a chip area becomes small, thereby enabling higher integration and larger structure. However, as compared to the current-induced magnetic field writing MRAM, a write time period tends to be longer (ex. 1 nano-second or more).
U.S. Pat. No. 6,834,005 discloses a magnetic shift register using a spin transfer. This magnetic shift register stores data by using domain walls in a magnetic material. In the magnetic material having multiple areas (magnetic domains), a current is introduced to pass through the domain wall thereby moving the domain wall. The magnetization direction of each of the areas is treated as a recorded data. The domain wall motion in a magnetic material is also reported in A. Yamaguchi et al., “Real-Space Observation of Current-Driven Domain Wall Motion in Submicron Magnetic Wires”, Physical Review Letters, Vol. 92, pp. 077205-1-4 (2004).
A domain wall motion MRAM using the domain wall motion by the spin transfer is described in Japanese Patent Publication JP2005-191032A and International Publication WO2007/020823A.
The MRAM described in JP2005-191032A includes a magnetization fixed layer in which magnetization is fixed, a tunnel insulating layer laminated on the magnetization fixed layer, and a magnetization recording layer laminated on the tunnel insulating layer. Here, since the magnetization recording layer includes a portion in which a magnetization direction can be switched and a portion in which a magnetization direction is not substantially switched, the layer is called not the magnetization free layer but the magnetization recording layer. FIG. 1 is a schematic plan view showing a structure of the magnetic recording layer in JP2005-191032A. In FIG. 1, the magnetic recording layer 100 has a rectilinear shape. Specifically, the magnetic recording layer 100 includes a connecting portion 103 which overlaps with the tunnel insulating layer and the magnetization fixed layer, constricted portions 109 which are adjacent to both ends of the connecting port ion 103, and a pair of magnetization fixed areas 101 and 102 which are adjacent to the constricted portions 104. The magnetization fixed areas 101 and 102 have fixed magnetization, in which the magnetization direction of the magnetization fixed area 101 is opposite to that of the magnetization fixed area 102. The MRAM further includes a pair of writing terminals 105 and 106 which are electrically connected to the pair of the magnetization fixed areas 101 and 102. By using the writing terminals 105 and 106, the current passing through the connecting portion 103, the pair of the constricted portions 104, and the pair of the magnetization fixed areas 101 and 102 of the magnetic recording layer 100 flows.
FIG. 2 is a schematic plan view showing a structure of a magnetic recording layer 120 of WO2007/020823A. In FIG. 2, the magnetic recording layer 120 has a U-shape. Specifically, the magnetic recording layer 120 includes a first magnetization fixed area 121, a second magnetization fixed area 122, and a magnetization switching area 123. The magnetization switching area 123 overlaps with a pinned layer 130. The first and second magnetization fixed layers 121 and 122 are formed so as to be extended to the y direction. The magnetization directions of the first and second magnetization fixed layers 121 and 122 are the same direction. On the other hand, the magnetization switching area 123 is formed so as to be extended to the x direction. The magnetization direction of the magnetization switching area 123 can be switched. Therefore, a domain wall is formed at one of a boundary B1 between the first magnetization fixed area 121 and the magnetization switching area 123 and a boundary B2 between the second magnetization fixed area 122 and the magnetization switching area 123. The first and second magnetization fixed areas 121 and 122 are connected to current supplying terminals 125 and 126, respectively. By using the current supplying terminals 125 and 126, a write current can be made to flow through the magnetization recording layer 120. The domain wall moves inside the magnetization switching area 123 based on a direction of the write current. According to this domain wall motion, the magnetization direction of the magnetization switching area 123 can be controlled.
However, in the current driven domain wall motion MRAM, there is concern that an absolute value of the write current tends to relatively increase. There are many reports observing the current driven domain wall motion including the foregoing Physical Review Letters, Vol. 92, pp. 077205-1-4 (2004). However, a current density of approximately 1×108 A/cm2 is required as a threshold for the domain wall motion. For example; it is assumed that a width and a thickness of a layer where the domain wall motion arises are 100 nm and 10 nm. In this case, the write current is 1 mA. In order to reduce the write current less than this value, it may be considered that the thickness should be thinner than before. However, in this case, it is known that the threshold current density required for writing further increase (e.g., see A. Yamaguchi et al., “Reduction of Threshold Current Density for Current-Driven Domain Wall Motion using Shape Control”, Japanese Journal of Applied Physics, vol. 45, No. 5A, pp. 3850-3853 (2006)).
On the other hand, as for an element using perpendicular magnetic anisotropy material, in which magnetic anisotropy is perpendicular to a substrate surface, for the magnetic recording layer, the following fact is experimentally indicated. That is, the threshold current density required for the current induced domain wall motion is smaller than that of an element using in-plane magnetic anisotropy material. For example, D. Ravelosona et al., “Threshold currents to move domain walls in films with perpendicular anisotropy”, Applied Physics Letters, Vol. 90, 072508 (2007) indicates that the threshold current density of approximately 16 A/cm2 order is observed. Further, S. Fukami et al., “Micromagnetic analysis of current driven domain wall motion in nanostrips with perpendicular magnetic anisotropy”, J. Appl. Phys. 103, 07E718 (2008) theoretically indicates that the threshold current density required for the current induced domain wall motion is smaller than that of an element using in-plane magnetic anisotropy material. Therefore, in the current driven domain wall motion MRAM, it is expected that the write current can be reduced by using the perpendicular anisotropy material for the magnetic recording layer.
In conjunction with the above-mentioned techniques, Japanese Patent Publication JP2006-73930A discloses a varying method of a magnetization state of magneto-resistance effect element using a domain wall motion, a magnetic memory element and a solid magnetic memory using the method. The magnetic memory element includes a first magnetic layer, an interlayer and a second magnetic layer. The magnetic memory element records information as magnetization directions of the first and second magnetic layers. The magnetic memory element records information by forming regularly magnetic domains with mutual anti-parallel magnetizations and a domain wall separating those magnetic domains in at least one of the magnetic layers and moving the domain wall in the magnetic layer so as to control positions of the adjacent two magnetic domains. The second magnetic layer may have magnetic anisotropy perpendicular to an in-plane direction.
As mentioned above, in the current driven domain wall motion MRAM, there is concern that an absolute value of the write current tends to relatively increase. Therefore, the inventors have studied to reduce the write current by using a perpendicular magnetic anisotropy material as the magnetization recording layer in the current driven domain wall motion MRAM.
FIGS. 3A and 3B are a plan view and a sectional view, respectively, showing a possible magneto-resistance effect element using the perpendicular magnetic anisotropy material. A magnetization recording layer 210 includes a magnetization switching area 213 and a pair of magnetization fixed areas 211a and 211b. Here, in FIGS. 3A and 3B, a symbol of an open circle and a dot, a symbol of an open circle and an x-mark, and a symbol of an open arrow show magnetization directions of areas (e.g., the magnetization switching area 213 and magnetization fixed areas 211a and 211b in FIGS. 3A and 3B) where the symbols are drawn.
The magnetization switching area 213 overlaps with a tunnel insulating layer 232 and a pinned layer 230 and have a function as a free layer. That is, the magnetization switching area 213, the tunnel insulating layer 232 and the pinned layer 230 configure a magnetic tunneling junction (MTJ). The magnetization fixed area 211a is arranged adjacent to one end of the magnetization switching area 213, and the magnetization fixed area 211b is arranged adjacent to the other end of the magnetization switching area 213. At the connecting portions between the magnetization switching area 213 and the magnetization fixed areas 211a and 211b, constricted portions 215 are arranged by applying the pinning potential forming method disclosed in JP2005-191032A. The direction of the fixed magnetization of the magnetization fixed area 211a and the direction of the fixed magnetization of the magnetization fixed area 211b should be opposite to each other. In addition, each constricted port ion 215 functions as pinning potential for the domain wall. The domain wall should be initialized so as to be a domain wall 212a or 212b at an area near the constricted portion 215.
Here, as shown in FIG. 2, if the magnetization recording layer has the in-plane magnetic anisotropy and the magnetization direction is an in-plane direction, by forming the magnetization recording layer to the U-shape, the magnetization directions of the magnetization fixed layers and the position of the domain wall can be easily initialized in an desired condition. However, as shown in FIGS. 3A and 3B, if the magnetization recording layer has the perpendicular magnetic anisotropy and the magnetization direction is a direction perpendicular to the in-plane direction, even by forming the magnetization recording layer to the U-shape, it is quite difficult to initialize the magnetization directions of the magnetization fixed layers and the position of the domain wall using an external magnetic field.
Further, in the domain wall motion MRAM, if the pinning potential forming method disclosed in JP2005-191032A is applied, the size of the constricted portion 215 is small as compared with that of the magnetization recording layer 210 as shown in FIGS. 3A and 3B. Therefore, there may be a case that the shape of the constricted portion 215 is deteriorated due to production tolerance. In that case, since the constricted portion 215 does not have the desired shape, the domain wall 212 cannot be pinned. Therefore, the magneto-resistance effect element cannot function as a magnetic memory cell. In addition, if the miniaturization of the magneto-resistance effect element advances and the width of the magnetization recording layer thereof may become narrow, it is very difficult to form the constricted portion 215 and there is a possibility to require a fabrication technique beyond the limitation of the lithography technique of the semiconductor processing.